A coaxial cable is an electrically conducting cable containing two or more conductors, each isolated from the others and running parallel to the others. Generally, such cables have a center conductor embedded in a dielectric, a woven or braided metallic shield surrounding the dielectric, and an outer insulating jacket which surrounds the shield. The center conductor carries a UHF or VHF radio frequency signal, while the braided conductor acts as an electromagnetic shield to prevent interference with the radio frequency signal.
A coaxial connector is a device for connecting a coaxial cable to another coaxial cable or to a different electronic medium, for example, a printed circuit board. In many instances, it is desirable to connect various types of signal conductors to a printed circuit board other than just a coaxial cable. For these cases, combination connectors are used which have both coaxial connectors and pin connectors arranged in an array in the same-connector housing. One of the conventional connectors of this type includes a D-subminiature housing having a female connector (receptacle) mateable with a male connector (plug). Other combination configurations are known, and it is evident that connectors which fit into a combination housing may be used individually for connection. The main function of such coaxial connectors is to provide a reliable and acceptable connection to coaxial cables of a given size.
In addition to providing a reliable and acceptable connection for a coaxial cable, it is another desirable attribute of a coaxial connector to provide for the maintenance of the characteristic impedance of the coaxial cable to which it is connected. In this regard, many previous coaxial connectors have had an upward limit of approximately 50 ohms. This is because the characteristic impedance Z of a connector is dependent upon the outer diameter of the inner conductor and the inner diameter of the outer housing, both of which are relatively fixed. In many instances, the outer housing of a coaxial connector is manufactured by a machining process and such process determines the characteristics of the material from which it is made, i.e., the material must be hard enough to chip during machining, and must be of a particular thickness to withstand the process. Because the outer diameter of such coaxial connectors is generally fixed by convention or standards, this produces a coaxial connector with a limitation on the inner diameter of the outer shell.
Further, many of the center conductors of coaxial connectors are pushed into a bore of a pre-formed dielectric member before assembly to the shell member of the coaxial connector. This process, because of the stiffness required for the center conductor, essentially defines the minimum outer diameter of the inner conductor. This again substantially limits the final impedance of the connector.
However, there are new applications for coaxial connectors which require such terminations to be of significantly higher impedance. For example, in the telecommunications and computer industry, a coaxial connection to a local area network or a telephone line should be terminated at approximately 75 ohms. This would create significant power loss if the standard 50 ohm connector is used.
One particularly advantageous coaxial connector for printed circuit boards is the receptacle end connector which is right-angled to a terminal end that allows a coaxial cable to be connected parallel to the plane of the printed circuit board. Such connectors have been suggested in the prior art, but have been inadequate in providing a low cost, inexpensive connector which can meet the impedance requirements of the present telecommunication and computer industries.
There have additionally been several problems in the manufacturing of coaxial connectors which increase their cost. Many of the coaxial connector shells are produced by a screw machining process which has a number of disadvantages. First, the screw-machined outer shell is inherently constructed of several piece parts which do not lend themselves to further simplified automated handling in the assembly process. Second, it is not readily adaptable between separate sizes of connectors and combination connectors. In fact, it is somewhat difficult to design and assemble separate retention means for the connector shells after they have been made.
Another difficulty is not being able to perform selective plating of contact metals on the connectors. Optimally, one would only plate noble contact metal in the places that the connector made a frictional fit with another connector. The present method is to barrel plate the entire connector shell, because selective plating of individual piece parts is even more expensive. However, significant plating material is wasted in this process.
Moreover, the screw-machined connector does not lend itself to sub-microminiaturization. New connectors will be required for denser circuit arrays in the future, and complete redesigns of the present connectors for materials and sizes will be required for machined connectors. It would be highly advantageous to find a process for making coaxial connectors which could be easily scaled to denser configurations without changing materials, processes, and design parameters.
The material, beryllium copper, which is generally used for making screw-machined connector shells, is relatively expensive and granular in structure. The hardness of the material must be suitable for ease of machining which limits its thickness. The spring finger contacts of a receptacle connector are formed by a secondary slitting or sawing operation on the shell. With this type of shell, it is difficult to calculate the stresses and the normal forces required for the proper contact engagement and the durability of the contact. One must generally rely on the spring properties of expensive beryllium copper and sometimes provide an additional heat treatment operation.